I believe they were always thinking about each other during the four years they were apart. I don't think Jack would ever be able to not try and find Ennis. Maybe Jack was worried that Ennis, now having married Alma and settled down into the life that was expected pf him, would be too afraid to see him again and thus refuse to meet him again, but Jack was willing to take that chance. Jack wouldn't let anything get in the way of finding Ennis again, he was resolved to do so. As for Ennis, of course his fear wouldn't allow him to actively go looking for Jack during that time, but at their reunion it's clear to see that Ennis had been waiting for Jack all that time. He left it up to Jack to find him, almost like he knew Jack would eventually find him, knowing Jack's personality, knowing his determination, he knew it would only be a matter of time.